The present invention relates generally to the use of a laser beam as a reference for controlling an excavating machine and, more particularly, to a method and apparatus for controlling such a machine to make linear digging strokes at a defined depth.
In recent years there has been increased use of laser beam projection systems in the construction industry. A laser beam rotated in a reference plane has been used to control the operation of various types of earth moving equipment, such as graders, scrapers, trenchers, and even excavating machines. Excavating machines will be used herein to refer to equipment for performing digging operations. Such equipment ranges from the largest self-controlled machines, referred to as "excavators," to the smallest machines, referred to as "backhoes," which may be attached to the rear end of a tractor. Due to the similar structure of such excavating machines, (i.e., an outreach boom pivotally connected to the machine, a downreach boom pivotally connected to the distal end of the outreach boom and a bucket pivotally connected to the distal end of the downreach boom) their normal digging stroke involves an arcuate movement of the downreach boom throughout the stroke. Unfortunately, not all excavating machine operators are sufficiently skilled that they can convert the arcuate movement of the downreach boom to a linear movement of the bucket parallel to a prescribed plane as is required to produce acceptable excavations.
One approach to applying laser control to an excavating machine, in this case a backhoe, to expand its capabilities and permit less skilled operators to dig flat trenches or the like, is disclosed in U.S. Pat. No. 4,231,700 issued Nov. 4, 1980 to Studebaker. The Studebaker system does not attempt to limit movement of the bucket to a planar stroke. Rather, the disclosed apparatus includes a detector mounted on a downreach boom which is kept in a fixed relationship with respect to a reference plane defined by a rotating laser beam. Although the detector is maintained at a fixed height, the cutting edge of the backhoe falls and rises during the digging stroke due to the pivoting action of the downreach boom. Thus, the bottom of a trench which is dug utilizing this system will not be flat.
A second approach is disclosed in U.S. Pat. No. 4,393,606 issued July 19, 1983 to Warnecke wherein an excavator uses a reference beam to permit the operator to control the bucket to make linear digging strokes. In the Warnecke system, a sensor is supported upon a mast which is in turn mounted directly on the upper part of the bucket. In Warnecke he sensor comprises a visually observable target such that an operator can control the excavator to maintain the laser beam centered upon the target to maintain a desired digging depth. Due to the nature of the bucket support, the orientation of the bucket remains constant throughout its digging stroke such that the desired target height of the beam striking the sensor is unchanged if the desired digging depth is maintained. Unfortunately, in an excavation of any depth, the Warnecke system requires placement of the laser source within the excavation and by locating the sensor on the bucket makes the sensor and the laser source readily susceptible to damage during the normal course of an excavation.
A third approach to laser control of the digging depth of an excavating machine is disclosed in U.S. Pat. No. 3,997,071 which issued Dec. 14, 1976 to Teach. In the Teach system, the angles between an outreach boom and horizontal, the outreach boom and the downreach boom, and the downreach boom and a line drawn to the digging teeth of the bucket are monitored and processed in accordance with trigonometric equations to provide a continuous signal and visual indication proportional to the depth of the digging teeth of the bucket relative to the mounting axis of the outreach boom. The absolute depth of the digging teeth of the bucket may be determined and displayed in the Teach system by measuring the absolute elevation of the mounting axis of the outreach boom relative to a reference plane defined by a rotating laser beam. In the Teach system, a beam sensor supported upon a movable mast is continually adjusted such that a defined section of the sensor is engaged by the rotating laser beam. Movements of the mast are monitored to determine the elevation of the axis of the outreach boom from which the absolute elevation of the digging teeth of the bucket can be determined and displayed.
A further advance of laser controlled excavating machines is disclosed in U.S. Pat. No. 4,129,224 which also issued to Teach on Dec. 12, 1978. In this system, the angles between the pivotally mounted elements of the excavating machine, in this case a backhoe, are monitored and applied to trigonometric equations such that the attack angle of the digging teeth of the bucket of the machine move parallel to a desired slope of an excavation throughout the digging stroke. Unfortunately, the mast structure and angle sensing apparatus must be extremely accurate to accurately control the depth of digging of an excavating machine. Hence, this system is relatively complicated and expensive.
Accordingly, there is a need for a simplified method and apparatus for operating an excavating machine in a manner such that linear digging strokes are made by the machine bucket at a defined depth.